In recent years, organic electro-luminescent (EL) elements have been arrayed in a matrix, which has been positively tested as a display panel. A driving method of this display panel employing the organic EL elements is disclosed as "a simple matrix method" in the Japanese Patent Application Unexamined Publication No. H06-301355.
FIG. 10 illustrates a structure and a driving method of a conventional display device.
In FIG. 10, the display device comprises display section 106, anode control circuit 107 and cathode control circuit 108. At each intersection of anodes "a1-am" and cathodes "c1-cn" arrayed in a matrix, light-emitting elements--formed of organic EL elements--"L1,1-Lm,n" are placed. The cathodes are scanned and driven at a given interval, and then the anodes are selectively driven being synchronized with this cathode-driving so that an arbitrary light-emitting element is selectively illuminated. Further, a reverse bias voltage or a voltage not more than a threshold value for illumination is applied to non-selected elements thereby avoiding erroneous lighting thereof (cross talk) due to leak current.
A driving method of the conventional display device is described hereinafter with reference to FIG. 10.
FIG. 10 illustrates a case where "L1,1" and "L2,1" among the light-emitting elements L1,1-Lm,n are selected to be lit. Anode lines "a1" and "a2" are coupled to current sources J1 and J2 by closing switches "Sa1" and "Sa2", and cathode line "c1" is coupled to ground potential (GND) by switch "Sc1", thereby running forward-bias-current to elements L1,1 and L2,1 and lighting these two elements.
Anode lines "a3-an" are coupled to ground potential by switches Sa3-Sam, and cathode lines "c2-cn" are coupled to power supply voltage Vcc by switches Sc2-Scn. Forward-bias-voltage produced both the ends of the two elements L1, 1 and L2,1 is referred to as Vf at lighting the two elements. Then the voltage applied to both the ends of non-lit elements takes either one of two values, i.e. "-Vcc" and "Vf-Vcc". The value of Vcc is set at a value so that the value of "Vf-Vcc" cannot be more than the threshold value of illumination, whereby non-selected elements are prevented from being erroneously lit.
However, this driving method produces two bias voltages at the non-lit elements. The elements having different bias voltages store different amount of charges in each parasitic capacitance of respective elements. Then when these non-lit elements are driven simultaneously, the elements biased at "-Vcc" light at a lower brightness than the elements biased at "Vff-Vcc". As a result, uneven brightness is observed between these elements.
The Japanese Patent Application Unexamined Publication No. H09-232074 teaches the following driving method which overcomes this problem: A reset period is reserved at switching the cathode to be driven, and during the reset period, switches Sa1, Sa2, and Sc2, Sc3, Sc4-Scn are switched so that these switches are coupled to ground potential as shown in broken lines in FIG. 10. This discharges charges stored in each parasitic capacitance of respective non-lit elements. This reset period can equal respective charges stored in each parasitic capacitance of the elements just before the elements are driven. As a result, uneven brightness due to a difference between stored charges can be avoided.
This method, however, discharges the stored charges once out of every parasitic capacitance at switching the cathodes to be driven, and charges every parasitic capacitance again at driving the elements, thereby consuming a large amount of power. The charges stored by applying a reverse-bias-voltage, in particular, do not contribute at all to lighting the element, i.e. they just waste electric power.
This power consumption due to the reverse-bias-voltage is detailed hereinafter in a more specific way. In the display device shown in FIG. 10, let us assume the following case: where
Parasitic capacitance of respective element: C (F) PA1 Power supply voltage of reverse-bias-voltage: Vcc (V) PA1 Frame frequency (a frequency for driving the cathodes in one cycle): Fv (Hz) PA1 (1) First, illuminate a first light-emitting element coupled to a first cathode; PA1 (2) Second, in order to illuminate a second light-emitting element coupled to a second cathode, run electric current into the second element. In this case, remove part of stored charges in the second element and leave charges in at least one light-emitting element other than the second element, then run electric current into the second element. PA1 (a) a plurality of light-emitting elements where the elements include: PA1 (b) an anode controller including: PA1 (c) a cathode controller including:
A static data is displayed on the display section, and a number of elements to be lit on a cathode "ca" (1.ltoreq.a.ltoreq.n) is "m.sub.on ", then the number of elements to which the reverse-bias-voltage Vcc is applied is (n-1).times.(m-m.sub.on), those elements are coupled to the cathodes except "ca" and coupled to anodes except the anodes of lit-elements.
Since those elements own parasitic capacitance "C" respectively, the energy "W" (J) supplied from the power supply to respective parasitic capacitances during the driving period of cathode "ca" is expressed as follows: EQU W=(1/2).multidot.C.multidot.(Vcc.sup.2).multidot.(n-1).multidot.(m-m.sub. on) (1)
The supplied energy "W" is discharged during the reset period, and charged by the power supply at the next scanning of the cathodes.
This control method discussed above can keep the non-selected elements at non-lit status. However, in an actual environment where this display device is used, external lights such as lamps and other light sources are also available. The elements reflect those external lights thereby producing reflection lights. The cathode lines are, in particular, formed of metal and thus produces a large amount of reflection lights. Under the strong external light such as sunlight, the difference between the illumination light and the reflection light becomes small, thereby lowering a contrast. As a result, pattern recognition of text data and the like becomes poor.
In order to overcome this disadvantage, a filter layer for limiting the external lights is often disposed on the surface of the display device. This measure decreases the influence of the external lights as well as increases an actual brightness responding to both of an attenuation factor in the filter layer and a desirable display brightness. A luminescent brightness of the conventional display device is determined with reference to a brightness visible enough even under intense external lights. Therefore, the display device illuminates with more brightness than it is required in a room or in the night where relatively weak external lights are available. The display thus becomes hard to see in a dark place, and consumes unnecessary power. This is a critical problem for display devices employed in battery-operated portable apparatuses among others.
As such, according to the conventional driving method, a power source for applying a reverse bias voltage supplies energy responsive to a number of non-lit elements at every scanning of cathodes. In this case a display pattern with a small number of lit-elements consumes a lot of power for charging/discharging each parasitic capacitance. This power basically does not contributes to lighting the elements, and just blocks the efforts of reducing power consumption.